The PI has found that the ventral midbrain has a transient organization into regularly spaced wedges of tissue distinguished by morphology and histochemistry. The wedges can even be visualized from the ventricular surface as a series of ridges and grooves. The pattern is remarkably similar to the rhombomeres of the hindbrain and they bear a number of anatomical markers that make them essentially as interesting for long term studies. The PI has called these structures midbrain arcs. He hypothesizes logically that these zones of tissue may constitute an early patterning mechanism for organizing neurons and their nuclei with differing neuronal fates as well as the axon projection patterns that arise from and connect with the arcs. He proposes a number of straightforward although technically difficult goals with which to study these various questions.